The field of invention is cable tensioning systems, and more particularly, a method and system for tensioning cables for concrete reinforcement.
Concrete reinforcement cable tensioning systems are used to form prestressed concrete structures, such as concrete floors, beams, and the like, that are reinforced with braided wire cables. To form the prestressed concrete structures, concrete is poured into a form having one or more of the cables laid therein, such that the cables extend through the concrete from one side of the structure to another side. After the concrete has partially cured, one end of each cable is secured to one side of the structure, and the other end of each cable is grasped by a concrete reinforcement cable tensioner which pulls on the cable to tension the cable and exert a compressive force on the concrete structure.
Concrete reinforcement cable tensioners known in the art typically have at least one hydraulic tensioning cylinder with a gripping mechanism fixed to the cylinder for grasping the cable, a crimping hydraulic cylinder that crimps a grommet onto the cable, and a housing, or frame, secured to the piston rods of the cylinders for bearing (directly or indirectly) against the side edge of the concrete slab so that a tension of a high magnitude can be exerted on the cable. A seat which mates with the grommet is typically cast into the side edge of the concrete where the cable comes out and the cable extends through the seat and through the grommet in the seat which only permits one way movement of the cable through the grommet.
Once the cable is tensioned, the crimping cylinder urges the grommet into the seat to crimp the grommet, such that the grommet engages the cable and tightens around it to prevent reverse movement and consequent reduction in the tensile force on the cable. Tensioning devices for performing this operation, seats and grommets are all well known. One exemplary concrete reinforcement cable tensioner is disclosed in U.S. Pat. No. 6,224,036 which is fully incorporated herein by reference and assigned to the assignee of the present invention.
A concrete reinforcement cable tensioner typically forms part of a concrete reinforcement cable tensioning system which includes a hydraulic unit that supplies hydraulic fluid to the hydraulic cylinders of the tensioner. The hydraulic unit typically includes a three position, four way manually operable hydraulic valve. The valve has a tensioning position which directs hydraulic fluid into the tensioning hydraulic cylinder to tension the cable, a hold position that maintains a constant tension on the cable, and a crimping position that directs hydraulic fluid into the crimping hydraulic cylinder to crimp the grommet onto the cable. A sequencing valve in the hydraulic unit releases tension on the cable by allowing hydraulic fluid to exhaust from the tensioning cylinder once the grommet has been crimped onto the cable.
In a known concrete reinforcement cable tensioning system, the sequencing valve exhausts hydraulic fluid from the tensioning hydraulic cylinder once the pressure in the crimping cylinder reaches a predetermined pressure. The predetermined pressure, however, is fixed regardless of the size of the cable being tensioned. If the diameter of the cable is large compared to the nominal cable diameter around which the concrete reinforcement cable tensioner was designed, the sequencing valve could release the tension in the cable before the grommet is securely crimped onto the cable.
If an improper crimping pressure is used, the cable can slip through the grommet and relieve the compressive force on the concrete structure, which provides a weaker concrete structure than intended. Likewise, if the diameter of the cable is small compared to the nominal cable diameter around which the concrete reinforcement cable tensioner was designed, the sequencing valve could fail to release the tension in the cable until long after the grommet is securely crimped onto the cable. As a result, the operation may result in too much tension in the cable, a damaged cable, or an operation that takes longer than is required which unnecessarily increases the cost of the operation.